vlmemory/svm: fix client detach from svm region

vlmemory/svm: fix client detach from svm region

Clients cannot know at svm region detach time if the shm backing files
have been recreated (e.g., if vpp restarts) and therefore should not try
to unlink them. Otherwise, terminating clients attached to previous
instantiations of a re-allocated region end up making the new instance
un-mappable by removing its backing file.

Change-Id: Idcd0cab776e63fd75b821bc9f0fac58217b9ccbe
Signed-off-by: Florin Coras <fcoras@cisco.com>